Frontend in English

<?php
/*
Plugin Name: Frontend in English
Description: Admin is displayed in the installed language, but the rest of the site is shown in default English
Version: 0.001
Author: Nikolay Bachiyski, Dero
Author URI: http://nikolay.bg/
Tags: translation, translations, i18n, admin, english, localization, backend
*/

function frontend_in_english_add_hooks() {
    add_filter( 'locale', 'frontend_in_english_locale' );
}
add_action( 'plugins_loaded', 'frontend_in_english_add_hooks' );

function frontend_in_english_locale( $locale ) {
    if ( frontend_in_english_should_use_english() ) {
        return 'en_US';
    }
    return $locale;
}

function frontend_in_english_should_use_english() {
    // frontend AJAX calls are mistakend for admin calls, because the endpoint is wp-admin/admin-ajax.php
    return frontend_in_english_is_frontend() || frontend_in_english_is_frontend_ajax();
}

function frontend_in_english_is_frontend() {
    return
        ! is_admin() && ! frontend_in_english_is_tiny_mce() && ! frontend_in_english_is_login_page();
}

function frontend_in_english_is_frontend_ajax() {
    return defined( 'DOING_AJAX' ) && DOING_AJAX && false === strpos( wp_get_referer(), '/wp-admin/' );
}

function frontend_in_english_is_tiny_mce() {
    return false !== strpos( $_SERVER['REQUEST_URI'], '/wp-includes/js/tinymce/');
}

function frontend_in_english_is_login_page() {
    return false !== strpos( $_SERVER['REQUEST_URI'], '/wp-login.php' );
}